Effects Of Intrinsic Deffects And Impurities On The Luminescence In ZnO

ZnO is considered to be the third-generation semiconductor with direct wide-band gap. The semiconductor ZnO has gained substantial interest in the research community in part because of its wide-bind gap(3.37 eV at room temperature) and large exciton binding energy (60 meV) which could lead to prospective applications in shortwave optoelectronics.ZnO is also considered to replace the GaN in the optoelectronics.ZnO based optoelectronic devices make great progress in recent years as a result of the development in the preparation of ZnO materials.However,at present the ZnO based devices have not reached the level of practical use.The development of ZnO meets its bottleneck where the fundamental issues in the ZnO are urgent to be investigated,such as intrinsic defects and impurities in ZnO.Defect and impurities are the dominant factors for the optical or electrical properties of ZnO.So in this dissertation the effects of defects and impurities on the optical and electrical properties of ZnO crystal are investigated.Considering these research background,the contents of this dissertation are listed as follows:Firstly,Zn diffusion method was employed to investigate optical and electrical properties resulted from the diffusion of Zn into ZnO crystals.Several important results are obtained:①Zn can diffuse into ZnO crystal and form Zn_i which is a shallow donor with ionization energy of～50 meV.Zn_i is considered as the candidate for residual n-type conductance.②UV emission intensity in ZnO is dramatically enhanced because of Zn_i.In the mean time Zn_o can be formed during Zn diffusing into ZnO and contribute to the visible emission peaked at 2.48 eV.③According to above experimental results,a new emission is proposed to explain the room-temperature UV emission,which is coupling emission of exciton with donor. Secondly,the effects of intrinsic defect on the visible emission in ZnO crystal are investigated by defect control through ambient and temperature variation.It is suggested that the visible emission in the ZnO is resulted from mutual effect of various intrinsic effects.Lastly,optical and structural properties of Ag-doped ZnO crystal are studied.It is suggested that Ag diffuse into ZnO and form Ag_o.A local vibrational mode and donor defect level related with Ag_o is observed.This dissertation is divided into six chapters.In chapter one,a comprehensive review is given on ZnO in crystal structure, fundamental physical properties,preparation method and prospective applications.In chapter two,the effects of Zn-doping on the intrinsic defects and ultraviolet emission in ZnO crystal are studied.It is found that the UV emission intensity in room-temperature PL spectra in the Zn-doped ZnO is enhanced by nearly an order compared with the as-grown ZnO.The emission mechanism for this phenomenon is explained.In chapter three,the temperature dependence of positions of the near-bind-edge emission peaks are investigated.It is found that there are three kinds of temperature dependence which may be resulted from different emission mechanism.A new emission mechanism,coupling emission of excitons with donor,is proposed to explain the UV emission at room temperature.In chapter four,the annealing control on the intrinsic defects and corresponding photoluminescence is investigated.It is suggested that different annealing condition could lead to different kinds of intrinsic defects and that the visible emission band in ZnO should result from mutual effects of various intrinsic defects.In chapter five,the Ag is introduced into the ZnO crystal by diffusion method.It is found that a local vibration mode appears in Ag-doped ZnO and that a level is introduced in the gap of ZnO.It is suggested that the Ag atom may diffuse into ZnO crystal and substitute on the oxygen site,forming Ag_o,which is a donor-like defect.In chapter six,a summary of this dissertation and suggestions for further study.